As part of a longitudinal, collaborative behavior genetics study in Colorado, behavior problems are studied in monozygotic and dizygotic twins at age five. The roles of genetic and shared environmental factors were assessed, based on multiple informants (mother, father, teacher). Boys were consistently reported to have externalizing problems (aggression, hostility, attentional problems and hyperactivity) than girls, but there are no sex differences for internalizing problems (anxiety, somatic complaints, depressed mood). Correlations between internalizing and externalizing problems were high, a pattern that suggests significant co-morbidity of problems in young children. Genetic influence on externalizing problems was consistently identified across measurement contexts. Both genetic and environmental influences on internalizing problems also were identified but less consistently. Children's social cognitions and observed patterns of coping with emotional challenges as potential differential predictors of externalizing and internalizing problems were examined. Children with more highly developed social understanding and regulated behavior patterns at ages 3 and 4, had fewer externalizing problems at age 5. Again the pattern was less consistent for internalizing problems. The indices of early social-emotional maturity were more characteristic of girls than boys. Associations between child behavior problems at age five and earlier child characteristics were more clearly identified when teacher rather than parent ratings of children's behavior problems were used. Possible rater bias in maternal and paternal reports of behavior problems may partially account for these differences and also artificially inflate estimates of both genetic and shared environmental influences on young children's behavior problems.